Photoconductive device having photoconductive layer containing hydroxyl radicals

ABSTRACT

A photoconductive device comprising a conductive substrate and a photoconductive layer applied on the conductive substrate, which photoconductive layer is made of amorphous silicon containing at least hydrogen, wherein the photoconductive layer contains hydroxy radicals. 
     Another photoconductive device comprising a conductive substrate, a photoconductive layer of amorphous silicon containing at least hydrogen and a surface protection layer applied on the photoconductive layer; wherein a dopant is added in the photoconductive layer at least near the interface with the surface protection layer, and the concentration of the dopant increases in the direction perpendicular to the interface and the surface protection layer has an optical energy gap larger than that of the photoconductive layer. 
     A further photoconductive device comprising a conductive substrate; a photoconductive layer of amorphous silicon applied on the conductive substrate and a surface protection layer of amorphous silicon applied on said photoconductive layer wherein the surface protection layer contains oxygen, and is doped with a IIIb element.

FIELD OF THE INVENTION

The present invention relates to a photoconductive device which issensitive to light and can be used, for example, as a xerographicphotoconductor and a photoconductive layer applied in apparatuses suchas a manuscript read-out apparatus.

PRIOR ART

Priorly, following photoconductive materials have been used toconstitute a photoconductive device: inorganic materials such asselenium, cadmium sulfide, and zinc oxide, and organic materials such aspolyvinyl carbazole and trinitrofluorenon.

FIG. 1 shows schematically a photoconductive device 101 composed of aconductive substrate 104 and a photoconductive layer 102 applied on thesubstrate 104.

However, those photoconductive materials do not necessarily satisfy allthe following properties required for a photoconductive device:photosensitivity, spectroscopic sensitivity, the SN ratio (lightresistance/dark resistance), durability and safety (danger) for a humanbody. Then, they have been applied each in optimum occasions by relaxingone or more requirements to some extent.

Recently, amorphous silicon (hereinafter referred to as a-Si, wherein"a-" means "amorphous") photoconductive device has been studied largelydue to advantages of high photosensitivity, high durability andharmlessness. However, a-Si photoconductive material has many points tobe improved further.

For example, the SN ratio is not high enough because the dark resistanceis low. Then, if an a-Si photoconductive layer is applied for axerographic photoconductor, sufficient surface potential cannot beobtained due to the low dark resistance, and the contrast of theelectric potential is not clear. Then, the image concentration has oftenbeen inappropriately low.

Furthermore, as an a-Si xerographic photoconductor is re-usedrepeatedly, the number of damages such as scratches increases andpoint-like defects happen to appear in a copy of an image. Repeatedoperations make an image in a copy dim gradually.

Especially, it should be noted that the surface of a-Si is unstable. Theunstability deteriorates environmental resistance, such as moistureresistance, as well as secular stability. If an a-Si xerographicphotoconductor is allowed to be out of operation for a long period, animage in a copy becomes dim.

It is an object of the present invention to provide a photoconductivedevice made from a-Si-based material and having improvedcharacteristics, especially high dark resistance, so as to make the S/Nratio (dark resistance/light resistance) large.

It is another object of the present invention to provide aphotoconductive device which is excellent especially in stability,environment resistance and secular stability.

It is a further object of the present invention to provide aphotoconductive device which has hard surface and adheres to thesubstrate well.

SUMMARY OF THE INVENTION

In accordance with the present invention, a new photoconductive deviceis provided which comprises:

a conductive substrate; and

a photoconductive layer applied on the conductive substrate, whichphotoconductive layer is made of amorphous silicon containing at leasthydrogen, wherein the new photoconductive layer contains hydroxyradicals.

In accordance with the present invention, another photoconductive deviceis provided which comprises:

a conductive substrate;

a photoconductive layer of amorphous silicon containing at leasthydrogen; and

a surface protection layer applied on the photoconductive layer;

wherein a dopant is added in the photoconductive layer at least near theinterface with the surface protection layer, and the concentration ofthe dopant increases in the direction perpendicular to the interface;and the surface protection layer has the optical energy gap larger thanthat of the photoconductive layer.

It is an advantage of the present invention to provide a photoconductivedevice, which can be used, for example, as a xerographic photoconductor.Especially, when used as a xerographic photoconductor, an image can bereproduced with high concentration and with high resolution, and ahalf-tone image can be obtained clearly.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplesand with reference to the accompanying drawings, in which:

FIG. 1 is a schematic cross-section of a photoconductive device;

FIG. 2 is a schematic cross-section of a photoconductive device with asurface protection layer;

FIG. 3 is a schematic cross-section of a photoconductive device with anundercoat layer;

FIG. 4 is a diagram of the deposition apparatus;

FIG. 5 is a graph of the light absorption of photoconductive devices;

FIG. 6 is a diagram of an apparatus for test electrophotographycharacteristics;

FIGS. 7-9 are graphs of physical properties of photoconductive devices;

FIGS. 10(a)-(e) are schematic energy diagrams of a photoconductivedevice;

FIG. 11 is a diagram of an electrophotography apparatus;

FIGS. 12(a)-(f) are graphs of the distribution of boron (solid line) andanother element such as oxygen, nitrogen and carbon (dashed line) in aphotoconductive device; and

FIGS. 13(a)-(f) are graphs of the distribution of phosphor (solid line)and another element such as oxygen, nitrogen and carbon (dashed line) ina photoconductive device.

DETAILED DESCRIPTION

One solution (shown in Example 1) to overcome the disadvantagesmentioned in the introduction is to add chemical modifiers to an a-Siphotoconductive layer. From the standpoint of the improvement ofenvironment resistance and secular stability, the addition of chemicalmodifiers, such as oxygen, nitrogen and carbon is appropriate.Unfortunately, the inventors found that the characteristics for aphotosensitive material cannot be improved. On the contrary, theinventors found that the addition of hydroxy radicals is effective.

Another solution (shown in Examples 2-8) to overcome the disadvantagesis to apply a surface protection layer or a surface blocking layer tothe surface of an a-Si photoconductive layer. A surface protection layer103 covers a photoconductive layer 102 as shown in a schematiccross-section of a photoconductive device 101 (FIG. 1), wherein aphotoconductive layer 102 is applied on a conductive substrate 104, anda surface protection layer 103 covers the photoconductive layer 102; thesurface protection layer 103 has a free surface 106. FIG. 3 showsanother schematic cross-section of a photoconductive plate 101 having asurface protection layer 103, wherein an undercoat layer 105 isinterposed between the substrate 104 and the photoconductive layer 102in order to improve the adhesion between them. The undercoat layer 105prevents not only the fall-off of deposited films from the conductivesubstrate 104, but also the injection of carriers from the conductivesubstrate 104.

However, a surface protection layer is liable to deteriorate propertiesimportant for a photosensitive material such as charge acceptance,photosensitivity and residual potential. If a surface protection layeris formed as an electrically insulating film (by decreasing thecomposition ratio x) in order to improve environment resistance andsecular stability, the photosensitivity deteriorates and the residualelectrical potential rises. Then, for example, photographic foregoinghappen in a copy. On the contrary, if the photoconductivity of a surfaceprotection layer is made higher (by increasing x), the environmentresistance and the secular stability deteriorate though the residualpotential decreases.

On the other hand, as will be explained in Examples 2-8, aphotoconductive device can be produced covered with a surface protectionlayer which does not deteriorate the photoconductor characteristics.

A photoconductive device can be produced as follows:

Amorphous silicon can usually be formed on a conductive substrate with adeposition process such as the glow discharge process, the sputteringprocess, the ion plating process and the vacuum deposition process.

A conductive substrate is made of stainless steel, aluminum, chromium,molybdenum, gold, iridium, niobium tantalum or an alloy of theabovementioned metals.

If a flexible conductive substrate is needed, an appropriate metal amongthe abovementioned metals is applied to a film sheet of synthetic resinby using the vacuum deposition process, the sputtering process, or thelaminating process. Next, a process of depositing a photoconductivelayer on the flexible conductive substrate follows.

In the glow discharge process which has been used in following Examples1-8, silicon hydrides such as SiH₄, and Si₂ H₆ are introduced in adeposition apparatus under vacuum, and are discharged under low pressureso that they are decomposed with the discharge energy and are depositedwith the discharge energy on a substrate placed in the depositionapparatus.

A process of adding chemical modifiers such as hydrogen, oxygen,nitrogen, carbon and hydroxy radical to a-Si are as follows.

Hydrogen will be added simultaneously at the deposition of a-Si becausethe starting materials for the deposition of a-Si are silicon hydridessuch as SiH₄, Si₂ H₆ ; the hydrides are decomposed with the discharge soas to form a-Si:H. In other words, a-Si deposited in this processcontains at least hydrogen. If it is necessary to add hydrogen moreeffectively, hydrogen gas will be introduced in the depositionapparatus.

In case of adding oxygen, the oxygen gas is introduced in the depositionprocess. Because the oxygen and gases such as SiH₄ react vigorously inthe gas phase, the former should be introduced via a path different fromthat of the other gases.

Nitrogen or carbon can be added to a-Si only by introducing nitrogen gasor a gas of nitride or carbide such as ammonia (NH₃), mathane (CH₄), andacetylene (C₂ H₂).

Dopants are added in order to control the type of carrier in a-Si.Doprng of accepters such as a IIIb element, boron, and of doners such asa Vb element, phosphor, makes the type of carriers the p-type and then-type, respectively. Boron or phosphor is conventionally doped byintroducing diborane (B₂ H₆) or phosphine (PH₃) in the depositionapparatus. The amount of the addition of the dopant is determinedaccording to the electrical and optical properties to be required.

FIG. 4 shows a deposition apparatus for the glow discharge process. Analuminum substrate 2 of diameter 140 mm and length 340 mm is secured ona drum heater 3 which can be rotated in a reaction chamber 1 with adrive motor 7. The surface of the aluminum substrate 2 has been washedsufficiently in a chlorocen supersonic washer and a steam cleaner (notshown). The drum heater 3 has the outer surface which coheres closelywith the inner surface of the aluminum substrate 2, and it heats thesurface of the aluminum substrate 2 homogeneously. The reaction chamber1 has a small window 5.

A pair of discharge electrodes 4, 4' are placed beside the aluminumsubstrate 3 symmetrically relative to the substrate 3, and a highfrequency power source 6 is connected to the electrodes 4, 4'.

The reaction chamber 1 can be evacuated via a valve 8 with a mechanicalbooster pump 9 and a rotary pump 10. A relief valve 11 is furnished inan evacuation pipe.

After opening valves 30-38, gases stored in gas cylinders 20-28 can beintroduced in the reaction chamber 1 through an auxiliary valve 12 underthe control of mass flow controllers 40-48, respectively. The gascylinder 20 contains silane (SiH₄), while the gas cylinder 21 is filledwith hydrogen. The gas cylinder 22 and 23 contain 400 ppm and 1% ofdiborane mixed with hydrogen, respectively, while the gas cylinder 24contains 600 ppm of phosphine mixed with hydrogen. The gas cylinder 25and 26 are filled with ammonia (NH₃) and methane (CH₄), respectively.The gas cylinders 27 and 28 contain silicon tetrafluoride (SiF₄) andcarbon tetraflouride (CF₄), respectively.

Oxygen gas in a gas cylinder 29 can be introduced through a differentline via a valve 39 and an auxiliary valve 13 under the control of an:ass flow controller 49.

Methyl alcohol is used in order to add hydroxy radicals to a-Si. Becausemethyl alcohol is liquid under room temperature, hydrogen gas suppliedfrom the gas cylinder 21 is introduced in a bubler 51, wherein thehydrogen gas bubbles through the methyl alcohol contained in the bubler51 and carries methyl alcohol gas to the reaction chamber 1 under thecontrol of a controller 52 which regulates the ratio of methyl alcoholto hydrogen.

A photoconductive layer 102 can be deposited on a conductive substrate104 as follows. The air in the reaction chamber 1 is evacuated via thevalve 8 with the mechanical booster pump 9 and the rotary pump 10, andthe aluminum substrate 2 is heated by the drum heater 3 until thetemperature of the surface of the aluminum substrate 2 increases to 250°C. and is kept constant thereafter.

Then, gases composed of silane, hydrogen and others to be mixed, forexample diborane, are allowed to flow to the reaction chamber 1 byopening the auxiliary valve 12 and the valves 30, 31 and 32. The setpoints of the mass flow controller 40, 41 and 42 are increased graduallyto predetermined values. The pressure in the reaction chamber 1 is keptat 1.5 Torr by controlling the opening of the valve 8.

Next, the high frequency power source 6 is switched on to apply avoltage of high frequency 13.56 MHz to the pair of discharge electrodes4, 4'. Thus, the glow discharge begins, and an a-Si film is deposited onthe aluminum substrate heated as mentioned above. The high frequencypower is controlled at 400 W during the deposition process.

On the deposition of an a-Si film, diborane or phosphine is added inorder to control the carrier type of the a-Si film. For example, theflow of diborane is controlled with the mass flow controller 42 so thatthe ratio of the concentration of diborane to that of silane becomes10⁻⁴. If the amount of diborane is high, the carrier type becomes p-typeor p⁺ -type, while if low, the carrier type becomes n-type or i-type. Asfor the dark resistance ratio and the photosensitivity, a very smallamount of oxygen, methane or ammonia is added in order to improve themto predetermined values.

After forming an a-Si layer, the high frequency source 6 is switchedoff, the valves 30, 31, 32 and 12 are closed, and the drum heater 3 isswitched off to cool down gradually.

After the aluminum substrate 2 is removed, carbon tetrafluoride gas inthe gas cylinder 28 is introduced in the reaction chamber 1 under thecontrol of the mass flow controller 26 in order to clean the reactionchamber 1.

PREFERRED EMBODIMENTS EXAMPLE 1

Electrical and optical properties of an a-Si photoconductive device canbe controlled by adding chemical modifiers such as oxygen, nitrogen andcarbon. However, the addition of those chemical modifiers cannotincrease both dark resistance and photoconductivity gain at the time;the improvement of one of both properties accompanies the deteriorationof the other.

A photoconductive device of double layer structure shown in FIG. 1 isformed wherein hydroxy radicals are added in the photoconductive layer102. Molecules which has a hydroxy radial are mixed with silane ordisilane and are introduced in the reaction chamber 1 in order to addhydroxy radicals to a-Si. Such molecules are, for example, alcohols suchas methyl alcohol and ethyl alcohol and fatty acids such as acetic acid.

It is possible to add a dopant to a-Si at the same time in order tocontrol the carrier type of the photoconductive layer.

If the molecule having a hydroxy radical is liquid at room temperature,it is necessary to use a mass flow controller of the liquid vaporcontrol type. If water is added, it is especially needed to control bothpurity and flow rate because of the large activity of water.

In this Example, because methyl alcohol is liquid at room temperature,carrier gas, that is, hydrogen gas from the gas cylinder 21 isintroduced in the bubler 29 which contains methyl alcohol. Thecontroller 49 controls the ratio of methyl alcohol to hydrogen as apredetermined ratio. The gas is introduced in the reaction chamber 1 aswell as silane, hydrogen, diborane and silicon tetrafluoride from thegas cylinders 20, 21, 22 and 27. The photoconductive layer 102 isdeposited with the glow discharge process as mentioned above, and thedeposirion rate is about 2.0 μm/hr. By depositing during eight hours, ana-Si film of thickness 16 μm are formed on a conductive substrate.

FIG. 5 plots the transmittance I of a-Si layer against the wave number(cm⁻¹) of the incident light.

The curve A shows a typical transmittance curve of a-Si containinghydroxy radicals. Three absorption peaks appear: a first peak around 650cm⁻¹ due to Si-H binding modes, a second peak due to Si-OH binding modesbetween 720 and 900 cm⁻¹ and a third peak between 2000 and 2100 cm⁻¹ dueto Si-H and Si-H² binding modes. The amplitudes of the three peaks varyaccording to the preparation conditions. On the other hand, the curve Bof a-Si containing oxygen has a second peak between 950 and 1050 cm⁻¹ ata different wave number range from that of the second peak of a-Sicontaining hydroxy radicals. The curve C shows for comparison thetransmittance of a-Si not containing hydroxy radicals and oxygen; It isclear that the second absorption peak vanishes. This graph shows clearlythat the addition of hydroxy radicals does nor deteriorate thephotoconductivity gain in spite of the increase in dark resistance aswill be shown later. Thus, the S/N ratio (dark resistance/lightresistance) becomes large with the addition of hydroxy radicals.

The a-Si containing hydrogen has the dark resistance as much as 10⁹-10¹⁰ Ω·cm. That is, the dark resistance is too low to be used for axerographic photoconductor so that the surface potential is not highsufficiently. Then, the contrast of the potential in an electrostaticlatent image becomes small, and the concentration of an image becomessmall. Thus, a copy lacks clearness.

On the contrary, if the a-Si containing at least hydrogen is doped withhydroxy redicals, the dark resistance becomes high and the SN ratio(dark resistance/light resistance) also increases. It is found that thea-Si containing hydrogen and doped with hydroxy radicals not only can beused practically, but also has remarkably excellent properties such asthe charge acceptance, the homogeneity and the durability.

The photoconductive layer 102 thus produced are examined with anelectrophotography testing apparatus shown in FIG. 6 as for thecharacteristics as a photoconductor. The decay characteristics of thesurface potential is measured with the apparatus wherein thephotoconductor are adhered on a drum 204. After a charger 201electrifies the rotating photoconductor, the drum 204 is stopped at thepredetermined timing, and at the same time the decay characteristics ofthe surface potential is detected with a potential sensor of the lighttransmittion type under the illumination of light through an opticalfiber 203.

At the measurements, the revolution of the drum 204 is controlled sothat the surface velocity of the photoconductor is 150 mm/sec, and thedc positive voltage applied on the sensitizer 201 is controlled so thatthe amount of the charges in the phoroconductor becomes 3×10⁻⁷coulomb/cm². Furthermore, the voltage applied to the source of light (ahalogen lamp of the rating of 24 V and 200 W) is controlled so that thelight intensity at the cutlet of the optical fiber 203 becomes 10μW/cm².

The data on the photoconductivity will be explained below. FIG. 7 showsthe time dependence of the surface potential (in Volt) of aphotoconductor P which contains hydroxy radicals according to thepresent invention and that of a photoconductor Q which contains nohydroxy radicals. The photoconductor begins to be illuminated at twosecond, and the surface potential drops rapidly after that. The timedependence of the surface potential up to 2 second, that is decay of thecharge in the darkness shows that the charge acceptance is remarkablyimproved in the photoconductor P according to the present invention.

FIG. 8 shows the charge acceptance (%) and the photosensitivity of aphotoconductor of thickness 16 μm against the amount of hydroxyradicals, which is expressed as a ratio L of the second peak to thefirst peak of the absorption intensity in the infrared absorptionspectra, that is, a ratio of the absorption intensity at 860 cm⁻¹ tothat at 650 cm⁻¹. The absorption intensity attains a maximum at 860 cm⁻¹in the second peak and at 650 cm⁻¹ in the first peak, respectively. Thesecond absorption peak between 720 and 920 cm⁻¹ is relevant to theexistence of hydroxy radicals.

It is clear that the charge acceptance increases with increase in L (theamount of hydroxy radicals). However, the photosensitivity decreases ifL becomes too large. Then, it is desirable that the ratio L is between0.1 and 2 for a photoconductor.

FIG. 9 shows another relation of both charge acceptance (%) andphotosensitivity of a photoconductor of thickness 16 μm to the amount ofhydroxy radicals which is expressed as a ratio L' of the second peak tothe third peak of the absorption intensity in the infrared absorptionspectra, that is, a ratio of the absorption intensity at 860 cm⁻¹ tothat at 2050 cm⁻¹. The absorption intensity attains a maximum at 2050cm⁻¹ in the third peak between 2000 and 2100 cm⁻¹.

It is desirable that the ratio L' (the amount of hydroxy radicals) isbetween 0.1 and 1.2 for a photoconductor. Similarly, the absorptionintensities of many samples have been measured. Then, it is found thatthe ratio of a broad peak between 720 and 900 cm⁻¹ due to the Si-OHbinding mode against that of a peak between 2000 and 2100 cm⁻¹ due tothe Si-H and Si-H₂ binding modes is desirably to be between 0.1 and 1.2for a photoconductor.

The inventors found that the characteristics as a xerographicphotoconductor cannot be improved by doping oxygen gas only, as willshown below. The characteristics of the infrared absorption of aphotosensitive film containing oxygen differ from those of aphotosensitive film containing hydroxy radicals. This is ascribable tothe difference of the chemical bonds of hydroxy radicals in the filmfrom those of oxygen atoms and the difference of the amount of thedopants.

In the following Examples, a photoconductive device having a surfaceprotection layer shown in FIGS. 2 and 3 is produced.

A surface protection layer should satisfy following requirments becauseit is applied on the surface of the photoconductive layer:

(a) The surface protection layer should not deteriorate thephotosensitivity of the photoconductive layer. In other words, it shouldhave small absorption coefficient of the visible light so as to transmitthe visible light to the photoconductive layer because thephotoconductive layer is composed of a material having largephotosensitivity or large absorption coefficient of the visible light.

(b) The surface protection layer should not deteriorate the chargeacceptance of the photoconductive layer. In other words, it should havehigh electrical resistance.

If a surface protection layer satisfies the abovementioned requirements(a), (b) as well as the environment resistance and the secularstability, a photoconductive device coated with the surface protectionfilm can have high photosensitivity and high charge acceptance as wellas excellent stability, environment resistance and secular stability.

In order to improve the environment resistance and the secularstability, a stable film of, for example, silicon carbide a-Si_(x)C_(1-x), silicon nitride a-Si_(x) N_(1-x), silicon oxide a-Si_(x)O_(1-x) (0<x<1) is used for a surface protection film. It is found thatif the composition ratio x becomes large, a film has a larger opticalband gap (a gap between energy bands of quantized states), becomesopaque, and becomes unstable, whereas if x becomes small, a film has asmaller optical band gap, becomes transparent for the visible light, andhas better environment resistance and secular stability.

Then, in order to improve the photosensitivity, it seems preferable toapply a surface protection layer to a photoconducting layer wherein theformer is composed of a material with smaller x and is transparent forthe visible light, have an optical band gap smaller than that of thelatter.

However, the matching of the energy bands between the photoconductivelayer and the surface protection layer is not good in such aphotoconductive device so that the photoconductive characteristicsbecome worse. FIG. 10(a) shows an energy level scheme in such asituation for a photoconductive device consisting of a metallicsubstrate 71, a photoconductive layer 72 and a surface protection layer73. Energy levels are occupied up to the Fermi level E_(F) in thesubstrate 71, while energy gaps E_(gp) and E_(gs) (E_(gp) <E_(gs)) existin the photosensitive layer 72 and in the surface protection layer 73,respectively. The bottom of the conduction band of the surface layer 73is higher by a difference a than that of the photoconductive layer 72,while the top of the valence band of the former 73 is lower bl adifference b than that of the latter 72. Then, if photocarriers(designated as + and - in FIG. 10(a)) are generated by the incidentlight in the photoconductive layer 72 near the interface with thesurface protection layer 73, they cannot move freely. Then, they cannotreach the free surface 106 and form space charges near the interface sothat the residual potential increases and the resolution of theelectrostatic latent image decreases.

On the contrary, if the photoconductive layer is made of a materialhaving a larger optical energy gap in order to improve the matching ofthe energy bands, the photosensitivity deteriorates. On the other band,if the surface protection layer is made of a material having smalleroptical energy gap, the charge acceptance, and the environmentresistance and the secular stability deteriorate.

The inventors can produce a photoconductive device coated with a surfaceprotection layer which satisfy the abovementioned requirements at thesame time even if the optical gap in the photoconductive layer issmaller than the counterpart in the surface protection layer. Thisproblem can be solved by controlling the type of the carriers near theinterface in the photoconducrive layer or by varying the Fermi energyappropriately so that photocarriers generated near the interface in thephotoconductive layer can reach the free surface of the surfaceprotection layer.

As shown in FIGS. 10(b), 10(c), if the concentration of acceptors (forexample, IIIb elements) is increased linearly or nonlinearly near theinterface, the conduction band and the valence band will become higher.On the other band, as shown in FIGS. 10(d), 10(e), if the concentrationof doners (for example, Vb elements) is increased linearly ornonlinearly near the interface, the conduction band and the valence bandwill become higher. Thus, the control of the concentration of the dopantnear the interface allows the energy bands to match near the interface.

Then, the photocarriers generated near the interface are allowed toreach the free surface smoothly due to the existence of the slope of theconduction and valence bands, even if the surface protection layer ismade of a material having a larger optical band gap. In practical use,for example, for a xerographic photoconductor, the applied electricvoltage shifts the energy levels and helps the matching of the energybands. Thus, no residual potential arises, and the resolution of anelectrostatic latent image becames high, and blurred images in a copy donot appear. Furthermore, the photoconductive material is superior in thecapability of sustaining charges, the environment resistance and thesecular stability.

EXAMPLE 2

A photoconductive device having a surface protection layer is producedas follows.

A photoconductive layer 102 is deposited as follows. Silane, hydrogenand diborane contained in gas cylinders 20, 21, 22 are allowed to flowconstantly in the reaction chamber 1 as mentioned above so that theratio of the concentration of diborane to silane is 10⁻⁴. Theapplication of high frequency voltage generates the glow discharge, anda-Si film is deposited on the conductive substrate 104.

After the deposition continues for 7.5 hours, the amount of diboraneintroduced in the reaction chamber 1 is increased gradually in order toincrease the concentration of positive carriers. That is, the controlknob of the mass flow controller 42 is kept on turning. After that, thehigh frequency power source 6 is turned off immediately. The ratio ofthe concentration of diborane to that of silane is 5×10⁻³ just beforethe turn off. Thus, a photoconductive layer 102 is formed on theconductive substrate 102 wherein the concentration of boron becomesmaximum around the surface.

As for the SN ratio and the photosensitivity of the a-Si photoconductivelayer 102, a person skilled in the art can easily control the valuesappropriately, for example, by adding a small amount of oxygen, methanecr ammonia.

Next, a surface projection layer 103 is formed with the glow dischargeprocess. The composition of the gases introduced in the reaction chamber1 is as follows. A predetermined amount of oxygen from the gas cylinder29 is introduced under the control of the mass flow controller 49 afteropening the valves 39 and 13. In order to make the flow rate of diboraneappropriate, diborane gas from the gas cylinder 23 is introduced underthe control of the mass flow controllers 43 after opening the valve 33as well as silane gas from the gas cylinder 20 under the control of themass flow controller 40 after opening the valve 30.

If the volume ratio of oxygen to silane is set between 0.5 and 2.0, asurface protection film 103 becomes more insulating, while if the ratiois set between 0.01 and 0.5, it becomes more photoconductive.

Table 1 shows electrophotography characteristics of a photoconductivedevice produced as mentioned above as well as those of a priorphotoconductive device wherein the concentration of boron in thephotoconduction layer 102 is constant.

                  TABLE 1                                                         ______________________________________                                                       Ex. 2  Prior art                                               ______________________________________                                        Surface potential                                                                              430 V    430 V                                               Residual potential                                                                              20 V    100 V                                               Environment resistance                                                                         excellent                                                                              excellent                                           Secular stability                                                                              excellent                                                                              good                                                Blurred images   excellent                                                                              not acceptable                                      ______________________________________                                    

The photoconducting device according to the present invention is foundto have very low residual potential. It is also found to have excellentcharacteristics for electrophotographic process such as the environmentresistance, the secular stability and blurred images in a copy.

The photoconductive device is used in the electrophotography process insuch an apparatus as shown in FIG. 11. A photoconductive device iscoated on a drum 308. In the electrophotography process, a first charger(6.0 kV) 301 applies a voltage on the drum 308 rotating clockwise. Animage on a manuscript is exposed through a lens 302 on the drum 308. Theexposed image is developed with a developer 303, and then it istransferred on a transfer paper 304. After the transfer, the drum 308 iscleaned with a cleaner 506, and it is discharged with a source of light306.

As shown in FIG. 3, an undercoat layer 105 can be interposed between thesubstrate 104 and the photoconductive layer 102. An undercoat layer onwhich a photoconductive layer will be applied may be composed of a-Sicontaining at least oxygen, nitrogen or carbon, and can be formed in asimilar process to that of forming a surface protection layer.

A photoconductive device produced in this Example and following Examples3 and 4 is especially useful when the photocarriers generated in thephotosensitive layer on the illumination should move freely. In orderfor a photoconductive material having positive dopants to makephotosensitive, the free surface should be charged positively.Otherwise, the photosensitivity characteristics of a photoconductivedevice cannot be shown fully. In the photoconductive device mentionedabove, photogenerated electrons pass easily the photoconduct layer underthe applied external field due to the abovementioned slope of the energydiagram (FIG. 10) near the interface with the surface protection layer,and cancel the charges in the free surface.

On the contrary, if the free surface is charged negatively,photogenerated holes cannot cancel surface charges due to the energybarrier at the interface, and becomes space charges. This has badinfluence on the electrophotography process.

FIGS. 12(a)-(f) show preferable examples of the consentration of boron(solid line j) and of oxygen (dashed line k) plotted against thedistance in the direction of the thickness of a photoconductive device,wherein X and Y represent the surface of the conductive substrate 104and the free surface 106, respectively.

As shown in FIGS. 12(a)-(f), the slope of boron concentration (j) aroundthe interface between the photoconductive layer 102 and the surfaceprotection layer 103 may be linear or be curved with a constantcurvature. The concentration of oxygen (k) may also have a slope aroundthe interface. As for the interface between the photoconductive layer102 and the undercoat layer 105 both concentrations may also have aslope.

In a photoconductive device shown in FIG. 12(a), the boron concentrationin the photosensitive layer 102 is constant except near the surfaceprotection layer 103 wherein it increases linearly, while the oxygenconcentration in the surface protection layer is constant. Thisstructure corresponds with that of the abovementioned Example 2.

A photoconductive device shown in FIG. 12(b) is the same with that shownin FIG. 12(a) except that an undercoat layer 105 containing both oxygenand boron is deposited on the substrate 104. In the undercoat layer 105,the concentration of boron is much larger than that of oxygen.

In a photoconductive device shown in FIG. 12(c) the concentration ofoxygen is much larger in both undercoat layer 105 and surface protectionlayer 103 than in the photoconductive layer 102. The boron concentrationis larger in both undercoat layer 105 and surface protection layer 103,while smaller in the photoconductive layer 102 except near the surfaceprotection layer 103 wherein it increases linearly.

In a photoconductive device shown in FIG. 12(d), the oxygenconcentration in the layer 105 and the surface protection layer 103decreases to zero continuously with a slope near the interfaces with thephotosensitive layer 102. The boron concentration in the layer 105 andthe surface protection layer 103 is larger than the boron concentration.The boron concentration decreases continuously to a constant value nearthe interface between the photoconductive layer 102 and the undercoatlayer 105, and increases again with a slope near the interface with thesurface protection layer 103.

In a photoconductive device shown in FIG. 12(e), the oxygenconcentration in the surface protection layer 103 decreases continuouslyto zero with a slope near the interface with the photoconductive layer102. The oxygen concentration in the undercoat layer 105 is much largerthan the boron concentration. The boron concentration in thephotoconductive layer 102 increases with a curve near the interface withthe surface protection layer 103, and it becomes larger than the oxygenconcentration in the surface protection layer 103.

A photosensitive device shown in FIG. 12(f) has the same structure as inFIG. 12(e) except that the surface protection layer 103 does not containboron.

EXAMPLE 3

A photoconductive device similar to that of Example 2 is produced asfollows. It contains nitrogen instead of oxygen in the surfaceprotection layer 103.

A photoconductive layer 102 is deposited on a conductive substrate 104in the same process as that explained in Example 2. As for the SN ratioand the photosensitivity of the a-Si photoconductive layer 102, a personskilled in the art can easily control the values appropriately, forexample, by adding a small amount of oxygen, methane or ammonia.

Next, a surface protection layer 103 is formed with the glow dischargeprocess. The composition of the gases introduced in the reaction chamber1 is as follows. A predetermined amount of ammonia from the gas cylinder25 is introduced under the control of the mass flow controller 45 afteropening the valve 35. ln order to make the flow rate of diboraneappropriate, diboran gas from the gas cylinder 23 is introduced underthe control of the mass flow controller 43 after opening the valve 33 aswell as silane gas from the gas cylinder 20 under the control of themass flow controller 40 after opening the valve 30.

If the volume ratio of anmonia to silane is set between 0.5 and 2.0, asurface protection film 103 becomes more insulating, while if the ratiois set between 0.01 and 0.5, it becomes more photoconductive.

Table 2 shows electrophotography characteristics of a photoconductivedevice produced as mentioned above as well as those of a priorphotoconductive device wherein the concentration of boron in thephotoconductive layer 102 is constant.

                  TABLE 2                                                         ______________________________________                                                       Ex. 3  Prior art                                               ______________________________________                                        Surface potential                                                                              390 V    380 V                                               Residual potential                                                                              20 V    100 V                                               Environment resistance                                                                         excellent                                                                              excellent                                           Secular stability                                                                              excellent                                                                              good                                                Blurred images   excellent                                                                              not acceptable                                      ______________________________________                                    

The photoconducting device according to the present invention is foundto have very low residual potential. It is also found to have excellentcharacteristics for electrophotographic process such as the enviromentresistance, the secular stability and the blurred images in a copy.

As shown in FIG. 3, an undercoat layer 105 can be interposed between thesubstrate 104 and the photoconductive layer 102. An undercoat layer onwhich a photoconductive layer will be applied may be composed of a-Sicontaining at least oxygen, nitrogen, and can be formed in a similarprocess to that of forming a surface protection layer.

FIGS. 12(a)-(f) show also preferable examples of the concentration ofboron (solid line j) and of nitrogen (dashed line k) plotted against thedistance in the direction of the thickness of a photoconductive device.

EXAMPLE 4

A photoconductive device similar to that of Example 2 is produced asfollows. It contains carbon instead of oxygen in the surface protectionlayer 103.

A photoconductive layer 102 is deposited on a conductive substrate 104in the same process as that explained in Example 2. As for the SN ratioand the photosensitivity of the a-Si photoconductive layer 102, a personskilled in the art can easily control the values appropriately, forexample, by adding a small amount of oxygen, methane or ammonia.

Next, a surface protection layer 103 is formed with the glow dischargeprocess. The composition of the gases introduced in the reaction chamber1 is as follows. A predetermined amount of methane from the gas cylinder26 is introduced under the control of the mass flow controller 46 afteropening the valve 36. In order to make the flow rate of diboraneappropriate, diborane gas from the gas cylinder 23, is introduced underthe control of the mass flow controller 43 after opening the valve 33 aswell as silane gas from the gas cylinder 20 under the control of themass flow controller 40 after opening the valve 30.

If the volume ratio of methane to silane is set between 0.5 and 2.0, asurface protection film 103 becomes more insulating, while if the ratiois set between 0.01 and 0.5, it becomes more photoconductive.

Table 3 shows electrophotography characteristics of a photoconductivedevice produced as mentioned above as well as those of a priorphotoconductive device wherein the concentration of boron in thephotoconductive layer 102 is constant.

                  TABLE 3                                                         ______________________________________                                                       Ex. 4  Prior art                                               ______________________________________                                        Surface potential                                                                              450 V    440 V                                               Residual potential                                                                              20 V    100 V                                               Environment resistance                                                                         excellent                                                                              excellent                                           Secular stability                                                                              excellent                                                                              good                                                Blurred images   excellent                                                                              not acceptable                                      ______________________________________                                    

The photoconducting device according to the present invention is foundto have very low residual potential. It is also found to have excellentcharacteristics for electrophotographic process such as the environmentresistance, the secular stability and the blurred images in a copy.

As shown in FIG. 3, an undercoat layer 105 can be interposed between thesubstrate 104 and the photoconductive layer 102. An undercoat layer onwhich a photoconductive layer will be applied may be composed of a-Sicontaining at least oxygen, nitrogen, or carben, and can be formed in asimilar process to that of forming a surface protection layer.

FIGS. 12(a)-(f) show also preferable examples of the consentration ofboron (solid line j) and of carbon (dashed line k) plotted against thedistance in the direction of the thickness of a photoconductive device.

EXAMPLE 5

A photoconducting material similar to that of Example 2 is produced asfollows. It contains nitrogen instead of oxygen in the surfaceprotection layer.

A photoconductive layer 102 is deposited on a conductive substrate 104in the same process as that explained in Example 2. As for the SN ratioand the photosensitivity of the a-Si photoconductive layer 102, a personskilled in the art can easily control the values appropriately, forexample, by adding a small amount of oxygen, methane or ammonia.

After the deposirion continues for 7.5 hours, the amount of diboraneintroduced in the reaction chamber 1 is decreased gradually in order tochange the carrier type to n-type. On the other hand, phosphor isprovided under the control of the mass flow controller 44 after openingthe valve 34. That is, the control knob of the mass flow controller 42and 44 are kept on turning during about thirty minutes in the directionof the decrease and of the increase in the flow rate, respectively.After that, the high frequency power source 6 is turned off immediately.The ratio of the concentration of diborane to that of silane is 5×10⁻³just before the turn off.

Thus, a photoconductive layer 102 is formed on the conductive substrate102 wherein the concentration of carbon becomes maximum around thesurface.

Next, a surface protection layer 103 is formed with the glow dischargeprocess. The composition of the gases introduced in the reaction chamberis as follows. A predetermined amount of ammonia from the gas cylinder25 is introduced under the control of the mass flow controller 45 afteropening the valve 35. In order to make the flow rate of phosphineappropriate, phosphine gas from the gas cylinder 24, is introduced underthe control of the mass flow controller 44 after opening the valve 34 aswell as silane gas from the gas cylinder 20 under the control of themass flow controller 40 after opening the valve 30.

If the volume ratio of ammonia to silane is set between 0.5 and 2.0, asurface protection film 103 becomes more insulating, while if the ratiois set between 0.01 and 0.5, it becomes more photoconductive.

Table 4 shows electrophotography characteristics of a photoconductivedevice produced as mentioned above as well as those of a priorphotoconductive device wherein the concentration of phosphor in thephotoconduction layer 102 is constant.

                  TABLE 4                                                         ______________________________________                                                       Ex. 5  Prior art                                               ______________________________________                                        Surface potential                                                                              -400 V   -400 V                                              Residual potential                                                                              -20 V   -100 V                                              Environment resistance                                                                         excellent                                                                              excellent                                           Secular stability                                                                              excellent                                                                              good                                                Blurred images   excellent                                                                              not acceptable                                      ______________________________________                                    

The photoconducting device according to the present invention is foundto have very low residual potential. It is also found to have excellentcharacteristics for electrophotographic process such as the environmentresistance, the secular stability and the blurred images in a copy.

As shown in FIG. 3, an undercoat layer 105 can be interposed between thesubstrate 104 and the phoroconductive layer 102. An undercoat layer onwhich a photoconductive layer will be applied may be composed of a-Sicontaining at least oxygen, nitrogen, ano can be formed in a similarprocess to that of forming a surface protection layer.

A photoconductive device produced in this Example and following Examples6 and 7 is especially useful when the photocarriers generated in thephotosensitive layer with the illumination should move freely. In orderto make a photoconductive material added with negative dopantsphotosensitive, the free surface should be charged negatively.Otherwise, the photosensitivity characteristics of a photoconductivedevice cannot be shown fully. In the photoconductive device mentionedabove, photogenerated holes pass easily the photosensitive layer underthe applied external field through the abovementioned slope of theenergy diagram near the interface with the surface protection layer, andcancel the charges in the free surface.

On the contrary, if the free surface is charged negatively,photogenerated electrons cannot cancel surface charges due to the energycarrier at the interface, and becomes space charges. This has badinfluence on the electrophotography process.

FIGS. 13(a)-(f) show preferable examples of the consentration ofphosphor (solid line 1) and of nitrogen (dashed line n) plotted againstthe distance in the direction of the thickness of a photoconductivematerial, wherein X and Y represent the surface of the conductivesubstrate 104 and the free surface 106 of a phtoconductive device,respectively.

As shown in FIGS. 13(a)-(f), the slope of phosphor concentration (1)around the interface between the photoconductive layer 102 and thesurface protection layer 103 may be linear or be curved with a constantcurvature. The concentration of nitrogen (m) may also have a slopearound the interface. As for the interface between the photoconductivelayer 102 and the undercoat layer 105 both concentrations may also havea slope.

In a photoconductive device shown in FIG. 13(a), the phosphorconcentration in the photoconductive layer 102 is constant except nearthe surface protection layer 103 wherein it increases linearly, whilethe nitrogen concentration in the surface protection layer is constant.

A photoconductive device shown in FIG. 13(b) is the same with that shownin FIG. 13(a) except that the undercoat layer 105 containing bothnitrogen and phosphor is deposited on the substrate 104 and that thephosphor is added only near the interface with the surface protectionlayer 103. This structure corresponds with that of the abovementionedExample 2 except the addition of nitrogen in the undercoat layer 105.

In a photoconductive device shown in FIG. 13(c) the concentration ofnitrogen is much larger in both undercoat layer 105 and surfaceprotection layer 103 than in the photoconductive layer 102. The phosphorconcentration is larger in both undercoat layer 105 and surfaceprotection layer 103, while smaller in the photoconduct layer 102 exceptnear the surface protection layer 103 wherein it increases linearly.

In a photoconductive device shown in FIG. 13(d), the nitrogenconcentration in the undercoat layer 105 and the surface protectionlayer 103 decreases to zero continuously with a slop near the interfaceswith the photoconductive layer 102. The phosphor concentration in theunderccat layer 105 and the surface protection layer 103 is larger thanthat in the photoconductive layer 102. The phosphor concentrationdecreases continuously to a constant value near the interface betweenthe photoconductive layer 102 and the undercoat layer 105, and increasesagain with a slope near the interface with the surface protection layer103.

In a photoconductive device shown in FIG. 13(e), the nitrogenconcentration in the surface protection layer 103 decreases continuouslyto zero with a slope near the interface with the photoconductive layer102. The nitrogen concentration in the undercoat layer 105 is muchlarger than the phosphor concentration. The phosphor concentration inthe photoconduct layer 102 increase with a curve near the interface withthe surface protection layer 103, and it becomes larger than thenitrogen concentration in the surface protection layer 103.

A photosensitive device shown in FIG. 13(f) has a similar structure tothat in FIG. 13(b). The phosphor concentration is curved near theihterface with the surface protection layer 103, and the nitrogenconcentration has a linear slope hear the interface.

EXAMPLE 6

A photoconductive device similar to that of Example 5 is produced asfollows. It contains carbon instead of nitrogen in the surfaceprotection layer 103.

A photoconductive layer 102 is deposited on a conductive substrate 104in the same process as that explained in Example 2. As for the SN ratioand the photosensitivity of the a-Si photoconductive layer 102, a personskilled in the art can easily control the values appropriately, forexample, by adding a small amount of oxygen, methane or ammonia.

Next, a surface protection layer 103 is formed with the glow dischargeprocess. The composition of the gases introduced in the reaction chamberis as follows. A predetermined amount of methane from the gas cylinder26 is introduced under the control of the mass flow controller 46 afteropening the valves 36. In order to make the flow rate of phosphineappropriate, phosphine gas from the gas cylinder 24 is introduced underthe control of the mass flow controller 44 after opening the valve 34 aswell as silane gas from the gas cylinder 20 under the control of themass flow controller 40 after opening the valve 30.

If the volume ratio of methane to silane is set between 0.5 and 2.0, asurface protection film 103 becomes more insulating, while if the ratiois set between 0.01 and 0.5, it becomes more photoconductive.

Table 5 shows electrophotography characteristics of a photoconductivedevice produced as mentioned above as well as those of a priorphotoconductive device wherein the concentration of boron in thephotoconduction layer 102 is constant.

                  TABLE 5                                                         ______________________________________                                                       Ex. 6  Prior art                                               ______________________________________                                        Surface potential                                                                              -440 V   -430 V                                              Residual potential                                                                              -20 V   -100 V                                              Environment resistance                                                                         excellent                                                                              excellent                                           Secular stability                                                                              excellent                                                                              good                                                Blurred images   excellent                                                                              not acceptable                                      ______________________________________                                    

The photoconducting device according to the present invention is foundto have very low residual potential. It is also found to have excellentcharacteristics for electrophotographic process such as the environmentresistance, the secular stability and the blurred images in a copy.

As shown in FIG. 3, an undercoat layer 105 can be interposed between thesubstrate 104 and the photoconductive layer 102. An undercoat layer onwhich a photoconductive layer will be applied may be composed of a-Sicontaining at least oxygen, and can be formed in a similar process tothat of forming a surface protection layer.

FIGS. 13(a)-(f) show also preferable examples of the concentration ofphosphor (solid line 1) and of carbon (dashed line m) plotted againstthe distance in the direction of the thickness of a photoconductivedevice.

EXAMPLE 7

A photoconductive device similar to that of Example 5 is produced asfollows. It contains oxygen instead of nitrogen in the surfaceprotection layer 103.

A photoconductive layer 102 is deposited on a conductive substrate 104in the same process as that explained in Example 2. As for the SN ratioand the photosensitivity of the a-Si phoroconductive layer 102, a personskilled in the art can easily control the values appropriately, forexample, by adding a small amount of oxygen, methane or ammonia.

Next, a surface protection layer 103 is formed with the glow dischargeprocess. The composition of the gases introduced in the reaction chamber1 is as follows. A predetermined amount of oxygen from the gas cylinder29 is introduced under the control of the mass flow controller 49 afteropening the valves 39 and 13. In order to make the flow rate ofphosphine appropriate, phosphine gas from the gas cylinder 24 isintroduced under the control of the mass flow controller 44 afteropening the valve 34 as well as silane gas from the gas cylinder 20under the control of the mass flow controller 40 after opening the valve30.

If the volume ratio of oxygen to silane is set between 0.5 and 2.0, asurface protection film 103 becomes more insulating, while if the ratiois set between 0.01 and 0.5, it becomes more photoconductive.

Table 6 shows electrophotography characteristics of a photoconductivedevice produced as mentioned above as well as those of a priorphotoconductive device wherein the concentration of boron in thephotoconduction layer 102 is constant.

                  TABLE 6                                                         ______________________________________                                                       Ex. 7  Prior art                                               ______________________________________                                        Surface potential                                                                              -460 V   -440 V                                              Residual potential                                                                              -20 V   -100 V                                              Environment resistance                                                                         excellent                                                                              excellent                                           Secular stability                                                                              excellent                                                                              good                                                Blurred images   excellent                                                                              not acceptable                                      ______________________________________                                    

The photoconducting device according to the present invention is foundto have very low residual potential. It is also found to have excellentcharacteristics for electrophotographic process such as the environmentresistance, the secular stability and the blurred images in a copy.

As shown in FIG. 3, an undercoat layer 105 can be interposed between thesubstrate 104 and the photoconductive layer 102. An undercoat layer onwhich a photoconductive layer will be applied may be composed of a-Sicontaining at least oxygen, nitrogen, or carbon, and can be formed in asimilar process to that of forming a surface protection layer.

FIGS. 13(a)-(f) show also preferable examples of the concentration ofphosphor (solid line 1) and of oxygen (dashed line m) plotted againstthe film distance in the direction of the thickness of a photoconductivedevice.

EXAMPLE 8

Another type of photoconductive device of a triple layer structure shownin FIG. 2 is formed as follows, wherein the concentration of the dopantsuch as boron is kept constant in the photoconductive layer while thesurface protection layer contains oxygen and boron.

A photoconductive layer 102 deposited on a conductive plate 104 in asimilar process to that in Example 2 during about eight hours. The boronconcenration is kept constant in the photoconductive layer 102. Then, asurface protection layer 103 is deposited on the photoconductive layer102. Eight samples are prepared by varying the amount of oxygen in thesurface protection layer.

Table 7 shows three parameters of the eight samples: the at% ratio(B/Si) of the concentration (at%) of boron atoms to that of siliconatoms, the volume ratio (O₂ /SiH₄) of oxygen gas to silane gas and thevolume ratio (B₂ H₆ /SiH₄) of diborance gas and silane gas introduced inthe reaction chamber 1.

The electrography characteristics, the environment resistance and thesecular stability of the eight samples are examined with anelectrophotography apparatus shown in FIG. 11.

                  TABLE 7                                                         ______________________________________                                        sample   B/Si                                                                 NO.      (at %)      O.sub.2 /SiH.sub.4                                                                     B.sub.2 H.sub.6 /SiH.sub.4                      ______________________________________                                        1         0.001      2.0      10.sup.-5                                       2        0.01        2.0      10.sup.-3                                       3        0.1         2.0      10.sup.-2                                       4        10.sup.-4   0.3      10.sup.-5                                       5        0.01        0.3      10.sup.-3                                       6        10.sup.-4   0.05     10.sup.-5                                       7        0.01        0.05     10.sup.-3                                       8        10.sup.-3   0        10.sup.-4                                       ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        sample                                                                              total    surface  residual                                                                             environment                                                                            secular                               NO.   thickness                                                                              potential                                                                              potential                                                                            resistance                                                                             stability                             ______________________________________                                        1     25 μm 450 V    350 V  ⊚                                                                       ⊚                      2     25 μm 440 V    80 V   ⊚                                                                       ⊚                      3     25 μm 430 V    20 V   ⊚                                                                       ⊚                      4     25 μm 300 V    50 V   ○ ○                              5     25 μm 300 V    20 V   ○ ○                              6     25 μm 280 V    30 V   Δ  Δ                               7     25 μm 280 V    20 V   Δ  Δ                               8     25 μm 280 V    20 V   X        X                                     ______________________________________                                         ⊚: excellent (the characteristic nearly deteriorates when      compared with the initial one.)                                                ○ : good                                                              Δ: fair                                                                 X: not acceptable                                                        

The environment resistance is examined by testing electrophotographycharacteristics repeatedly under high temperature and high moisture:thus, the environment resistance shown in Table 8 also means thestability. The secular stability is tested by comparing theelectrophotography characteristics with the initial one when eleven dayshave passed after the preparation of the sample.

Table 8 shows that samples No. 1, No. 2 and No. 3 are superior among theeight samples. Thus, it is clear that the surface protection layercontains much oxygen has excellent environment resrstance and secularstability. On the other hand, if the surface protection layer containslittle oxygen, the surface potential, the environment resistance and thesecular stability become worse.

Furthermore, the addition of boron decreases the residual potential. Theat% ratios B/Si of the samples No. 1 to No. 3 are 0.001, 0.01 and 0.1,respectively. Thus, it is preferable that the at% ratio B/Si is in therange between 10⁻¹ and 10⁻³.

The amount of oxygen and the at% ratio B/Si can be selected fordesirable values easily by a person skilled in the art.

As for the ratio B₂ H₆ /SiH₄, if the ratio is small, the residualpotential increases undersirably, whereas if the ratio becomes large,the residual potential decreases preferably.

In the abovementioned samples, boron is doped in the surface protectionlayers. However, it is found that other IIIb elements can be substitutedfor boron.

Furthermore, it is found that the characteristics can be improvedremarkably if the thickness of the surface protection layer is between0.01 and 4.0 μm.

This invention may be practiced or embodied in still other ways withoutdeparting from the spirit or essential character thereof. The preferedembodiments described herein are therefore illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims and all variations which come within the meaning of the claimsare intended to be embraced therein.

What is claimed as new:
 1. A photoconductive device comprising:aconductive substrate; and a photoconductive layer applied on theconductive substrate, which photoconductive layer is made of amorphoussilicon containing at least hydrogen; wherein the photoconductive layercontains hydroxy radicals.
 2. A photoconductive device according toclaim 1, wherein the intensity of a broad absorption peak at 860 cm⁻¹ inthe infrared absorption spectra of said photoconductive layer is from0.1 to 2.0 relative to the intensity of an absorption peak at 650 cm⁻¹.3. A photoconductive device according to claim 1, wherein the intensityof a broad absorption peak at 860 cm⁻¹ in the infrared absorptionspectra of said photoconductive layer is from 0.1 to 2.0 relative to theintensity of a peak at 2050 cm⁻¹.